Visualising vision correction

I’ve been trying out contact lenses for the first time. Multi-focal lenses provide different focal lengths to the eye at once, and you can have different prescription lenses in each eye (as long as they don’t differ by too much).

This means the brain is getting signals from the eyes, each providing potentially multiple focal lengths, and learns to combine them to reduce blur. It’s interesting and I wanted to be able to visualise how that works, so I made this interactive simulator with the help of Gemini. It shows a heatmap (green is sharp, red is blurry) over distance, comparing uncorrected vision with modern multi-focal lenses. Try it out! All the calculations happen locally within your browser.

Why do the rows look the way they do? Multi-focal contact lenses trade absolute sharpness for a wider range of vision. When seeing an object in sharp focus without corrective lenses, all of the light entering your eye gets focused at a single sharp point. If you can see an object in perfect focus, the light is converging to a single point on the retina.

With modern contact lenses this same amount of light is focused into multiple points: some might converge early (in front of the retina) or late (would be focused behind the retina). This is why the graph doesn’t show true green for the contact lenses. It’s the trade-off.

This next interactive simulator shows what’s actually happening to the rays of light within the eye, explaining why the focus heatmap above has dips in between areas of very good focus.